1. Field of the Invention
The invention relates to projecting images.
2. Description of the Related Art
Referring to FIG. 1, light projectors are used, for example, to display images on large surfaces, such as movie or television screens. In a front projection system 20, an image beam 24 is projected from an image source 21 onto the xe2x80x9cfrontxe2x80x9d side of a reflection-type angle transforming screen, which then reflects the light toward a viewer 27 positioned in front of the screen. In a rear projection system, the image beam is projected onto the xe2x80x9crearxe2x80x9d side of a transmission-type angle transforming screen 26 and transmitted toward a viewer 29 located in front of the screen.
Due to the physical properties of light, the size of the image beam 24 when it reaches the screen 26 depends upon the magnification rate of the image source 21 and the distance between the screen 26 and the image source 22. To efficiently utilize the screen 26, the image beam 24 should just fill the entire height S and width (not shown) of the screen 26. In FIG. 1, the image beam 24 fills the screen 26 when the source 22 is at an appropriate distance D from the screen 26, i.e., when the center of the image beam 24 follows an optical path 28 of length D. The optical oath length D depends upon the screen height S and the magnification rate of the image source. Any non-transparent object placed in the path of the image beam 24 typically will obstruct the image beam 24 and form a shadow on the screen 26.
Referring to FIG. 2, designers have reduced the size of the typical image projection system 30 by xe2x80x9cfoldingxe2x80x9d the optical path of the projected image beam to reduce the apparent projection length L of the system 30. The xe2x80x9cfoldedxe2x80x9d system 30 includes one or more mirrors 34, 36 placed at strategic points along the optical path to redirect the projected image and to form multiple optical subpaths D1 D2, D3. The overall system configuration in this optical system 30 is more rectilinear than the conical system configuration of FIG. 1.
In the folded image projection system 30, assuming that the image source 40 and the screen 32 are similar to those in FIG. 1, the image 38 fills the screen 32 entirely when the mirrors 34, 36 are positioned such that the combined length of the optical subpaths D1 D2, and D3 equals the optical path length D in FIG. 1, even though the apparent projection length L is less than D.
An xe2x80x9cextra-foldedxe2x80x9d projection display system includes a selectively reflective material (e.g., a linear reflecting polarizer) placed immediately behind the system""s imaging screen. The display system includes an image projector that projects an image beam containing light of a predetermined linear polarization toward the imaging screen. The linear reflecting polarizer reflects the light in the image beam away from the screen. The reflected image beam then encounters a xc2xc-wavelength achromatic retarder which converts the linear polarization to circular polarization. The image beam next hits a mirror that reflects the light back through the xc2xc-wavelength achromatic retarder, which converts the circular polarization back to linear polarization, with the polarization director rotated 90xc2x0 from the original polarization director. The linear reflecting polarizer then allows the light to pass through to the image screen.
Embodiments of the invention may include the following features. The system may include a powered optical element to further increase the xe2x80x9cfoldingxe2x80x9d of the image beam, change the magnification rate, provide distortion correction, or optimize packaging. The image beam may be folded even further by allowing it to reflect from the mirror twice and to pass through the achromatic retarder four times. The mirror may be positioned between the image source and the imaging screen.
Advantages of the invention may include one or more of the following. An optical element placed immediately behind the screen in a projection imaging system may act as a xe2x80x9cmirrorxe2x80x9d that increases the amount of folding that the light undergoes and therefore may reduce the apparent projection length, or depth, of the system to a greater extent than previously possible. The image source may be placed behind all optical elements in the display system, further reducing the size of the system.
Other advantages and features will become apparent from the following description and from the claims.